1. Field of the Invention
The present invention relates to a method of providing a seed diamond powder based on a seeding process which permits obtaining a thin synthetic diamond film, and simultaneously solve restrictions on the material for the synthetic diamond film forming surface (substrate surface).
The present invention makes it possible to synthesize a synthetic diamond film at a high nucleation density on various substrates such as oxide-based ceramics including SiO.sub.2, non-oxide-based ceramic including SiC, and metals. A synthetic diamond on a substrate is applicable for various uses utilizing properties of diamond. Typical examples of such uses include a protective film for a transparent substrate, an electron emitter for a field emission display, an SAW (surface acoustic wave) filter utilizing the higher sound velocity in diamond, a membrane for X-ray lithography, and a semiconductor device.
2. Description of the Background
Unless the substrate is pretreated, a synthetic diamond cannot be nucleated at a high nucleation density in the initial stage of vapor deposition, and it has been difficult to obtain a film-shaped diamond. To solve this problems trials have been made to improve the nucleation density in the form of, for example, the scratching process (improving the nucleation density by providing flaws on the substrate surface), and the seeding process (dispersing diamond powder in an aqueous solution or methanol and coating the same). The in-situ process is reported to be capable of improving the nucleation density by applying a negative bias to an Si substrate, and causing C atoms to be present on the substrate surface (a kind of implanting).
For the purpose of adhering seed diamond powder to a substrate, more specifically, it has been the usual practice to pre-treat by immersing an Si wafer in a dispersed solution prepared by dispersing a seed diamond powder having a particle size smaller than 2,000 nm in a methanol solution at a rate of 0.6 g/l, thereby causing the seed diamond powder to adhere to the substrate surface.
There is also known a technique to use synthetic diamond powder as a seed material powder, and as disclosed in Japanese Unexamined Patent Publication No. H09-25110, with a view to removing surface impurities, make a composition adjustment through washing with sulftric acid, nitric acid, potassium nitrate or chromic acid, thereby achieving bonding distribution of polar groups comprising hydroxyl groups (OH) or carbonyl groups (C.dbd.O) on the resultant surfaces of the seed diamond powder particles.
The scratching process, however, involves problems such as a damage to the substrate surface, limiting the substrate materials to Si, Mo, Pt and Ir, and a nucleation density lower than in the seeding process. Application of bias is used only for a few kinds of substrates, such as an Si substrate.
In the conventional seeding process, furthermore, applicable substrates are limited to Si and the like, and non-uniform seeding causes blurs in the vapor-deposited diamond film. Thus these defects have not as yet been overcome.
Applications of synthetic diamond films has therefore been limited because of the limitation on substrate materials. Unavailability of a sufficiently high nucleation density has led to defects in thick film-shaped diamond films. This is not desirable since this exerts an adverse effect on flatness of the formed synthetic diamond film and properties of the film itself.
SUMMARY OF THE INVENTION
To solve the aforementioned problems, the present invention provides a method of causing polar and non-polar groups to be coexistently distributed on the surfaces of seed diamond powder particles and seeding this seed diamond powder onto a synthetic diamond film forming surface (substrate surface), so that this seed diamond, excellent in adhesion, adheres to the substrate surface at a high density, thus giving a polycrystal diamond thin film of a high nucleation density free from blurs on the substrate surface.
More specifically, polar groups comprising hydroxyl group (OH), carbonyl group (C.dbd.O), aldehyde group (CHO) and/or carboxyl group (COOH) are surface bonded onto the surfaces of the diamond powder particles by applying a stirring treatment for a prescribed period of time in a treatment solution, such as a hydrogen peroxide/aqueous ammonia solution, an aqueous sulfuric acid/hydrogen peroxide solution, an aqueous aldehyde solution, or an aqueous acetic acid solution. Then, non-polar group comprising one or more selected from alkyl group (C.sub.n H.sub.2n+1) phenyl group (C.sub.6 H.sub.5), vinyl group (CH.dbd.CH.sub.2), acetyl group (CH.sub.3 CO), alkoxyl group (OC.sub.n H.sub.2n+1) and methacryl group (CH.sub.2 .dbd.CCH.sub.3) are surface-bonded by applying a stirring treatment for a prescribed period of time in a treatment solution such as a polyoxyethylenealkylether-containing solution, a triphenylsilanol-containing solution, a trimethoxyvinylsilane-containing solution, or a triethoxymethacrylsilane-containing solution. The resultant seeding diamond powder on which polar and non-polar groups are surface-bonded and coexistently distributed, brings about an excellent adhesion to the synthetic diamond film forming surface (substrate surface), adheres at a high density, and serves as diamond nucleation sites.
However, in order to cause the diamond to adhere to the substrate without blurs, particularly when applying the diamond to any of various substrates, as in the present invention, an appropriate dispersant should be selected. In order to reduce blurs occurring during seeding (causing blurs after diamond vapor deposition), it is necessary to take account of wettability to the substrate and the drying (vaporizing) rate, and a dispersant suitable for the seed diamond powder should be selected.
A small dispersion ratio of the seed diamond powder relative to the dispersant cannot improve adhering density of seeding. With a large ratio, on the other hand, a part of the seed diamond powder precipitates in the dispersed solution, thus making it difficult to obtain a dispersed solution of a high concentration. The concentration should therefore preferably be within a range of from 0.01 to 3.0 g/l, or more preferably, from 0.03 to 2.0 g/l.
Dispersants can be selected from any liquid as long as it does not react with the substrate or the diamond powder, and wets the substrate surface. For example, they are alcohols, such as methanol, ethanol and 2-propanol; ketones, such as acetone; ethers, such as diethyl ether; aldehydes, such as acetaldehyde; and esters, such as ethyl acetate.
This permits achievement of a very high film forming rate of the synthetic diamond film in the initial stage of vapor deposition.